The present invention relates to resin applied-copper foil which is capable of manufacturing a printed wiring board without employing component materials of an insulating layer such as a prepreg, and to a copper-clad laminate and a printed wiring board which are excellent in flame retardancy.
A copper-clad laminate used for manufacturing a printed wiring board which is widely used in electric and electronic products is generally manufactured by employing the so-called prepreg, the prepreg being made by the impregnation of a glass cloth, kraft paper, a nonwoven glass fabric or the like with a thermosetting resin such as phenol resin or epoxy resin in order to make the resin in a semi-cured state, and by sticking and laminating the copper foil onto one side or both sides of the prepreg.
Further, in case of manufacturing a copper-clad laminate having three layers or more which is categorized as a multilayer printed wiring board, the copper-clad laminate is manufactured by once performing the conductive circuit formation on a surface of the copper-clad laminate to make an internal layer base material, and by sticking the copper foil onto the both sides of the internal layer base material via the prepreg, for example.
In recent years, with the downsizing and greater packaging density of the printed wiring board, it has become common to provide fine blind holes, i.e., via holes, on a surface of the printed wiring board. As a method for forming this via holes, laser beam machining or plasma machining, for example, may be used. At this time, in a state that inorganic components like glass fibers are contained as frame materials in an insulating layer of the copper-clad laminate (for example, this state corresponds to a case of an insulating layer being comprised of a glass-epoxy prepreg which is referred to as an FR-4 base material.), the inorganic components as framework materials will affect the processability despite the ease of sublimation and evaporation of the resin component which are caused by the laser beam or plasma. Consequently, in order to allow uniform perforating, the insulating layer is frequently fabricated only with the resin components without employing the framework materials, to be used. Also, nonexistence of the frame materials results in a capability of eliminating influences of unevenness or ridges formed at a surface of the frame materials on a flatness of the copper-clad laminate, so that an extremely flat surface can be obtained. For example, as for the above described FR-4 substrate, cloth grains of the glass cloth which is a framework material of the insulating layer develop on a surface of outer layer copper foil of the copper-clad laminate after being subjected to the hot press molding, so that in case of fabricating a thin resist layer such as a liquid resist, variations occur in a thickness of the resist layer, which consequently deteriorates a width precision of an etched circuit. Therefore, these problems can be resolved by eliminating the framework materials.
In case of fabricating an insulating layer of a copper-clad laminate without including framework materials, manufacture of a multilayer copper-clad laminate is performed by any one of the following methods: (1) a method characterized in that a liquid resin is coated on a surface of an internal layer substrate and copper foil is stuck onto the resin, (2) a method characterized in that a resin film being comprised of a thermosetting resin in a semi-cured state is sandwiched between an inner layer substrate and copper foil to be stuck thereto, in order to perform thermoforming, (3) a method wherein resin applied-copper foil, i.e., copper foil having a resin layer formed thereon, is directly stuck to a surface of an inner layer substrate, or the like. Then the resultant product undergoes formation of the so-called outer layer circuits or formation of via holes, to be processed into a printed wiring board.
The above described method (1) has some problems, that is, it is difficult to more precisely and uniformly coat the liquid resin on a surface of the inner layer substrate on which inner layer circuits are already formed and uneven configurations are also formed, and in case of performing interlayer conducting plating when forming through holes or the like, it takes much time for grinding and removing the resin content which is deposited on an inner wall of the perforation or hole portion. And the resin film used in the above described method (2) is manufactured by coating a resin composition on a plastic film, however, costly plastic films to be discarded after its usage become enormous and thus a large amount of wastes are generated, so that from the view point of the environmental protection, this method is unacceptable in the present society wherein the environmental protection is taken very seriously.
Therefore, considering from the various view points, we have reached a conclusion that it is most advantageous to use resin applied-copper foil, and the above described method (3) is becoming widely and commonly pervasive. As a resin content used for the resin applied-copper foil, epoxy resins are frequently used. As for this resin applied-copper foil, the inventors have already proposed various resin applied-copper foil which are favorable for manufacturing the copper-clad laminate used for the printed wiring boards as a result of our intensive studies, and have widely contributed to the manufacture of high density printed wiring boards because manufacturing the printed wiring boards with the use of this resin applied-copper foil allows easy forming of via holes or the like employing the laser processing method as well as forming of stable fine pitch circuits.
The copper-clad laminate manufactured by the use of such a resin applied-copper foil, when used for the printed wiring board, has practically satisfiable heat resistance, electric characteristics, and chemical resistant characteristics and has been widely supplied to the market.
As a resin used for the formation of resin layers of resin applied-copper foil which has conventionally been supplied to the market, epoxy halide resins or halogen based flame retarders are used in order to impart the flame retardancy. As for the demands for flame retardancy, it is required also from a viewpoint of the PL Standards or the like to meet a certain criterion such that the ignition will not occur for the purpose of safety in electronic and electric equipment at the time of their usage.
As the criteria for this flame retardancy, there are various quality standards for the printed wiring boards, and each of home electrical products manufacturers and industrial electronics manufacturers etc. may often adopt its original criterion. Among others, a criterion defined in section 18 of the UL 796 which is also commonly referred to as xe2x80x9cthe UL Standardsxe2x80x9d, defining whether an item can be designated as a subject matter of insurance by the U.S. insurance company, has become a de facto standard as a practical global standard, and this criterion is one of the very important quality standards for the printed wiring boards. An evaluation test of the flame retardancy is conducted for a substrate from which noncombustible copper foil is removed after a copper-clad laminate is manufactured. Therefore, a problem is not a flame retardancy required for the resin applied-copper foil itself, but a flame retardancy required for the copper-clad laminate from which the copper foil is removed, that is, the flame retardant characteristics of the resin layer of the resin applied-copper foil which remains on the substrate side becomes a problem.
However, halogen elements added to the resin used for formation of the resin layers of the resin applied-copper foil has been reported to have an opportunity to generate toxic compounds during the combustion treatment after being discarded, so that it has been required to develop resin compounds and resin applied-copper foil etc. which are free of halogen based flame retarders.
In order to try to solve this problem, as an alternative to the halogen based flame retardar, it has been proposed to add inorganics such as phosphorous compounds represented by red phosphorous or phosphoric ester and metal hydroxides to the resin.
On the other hand, it has been pointed out that water resistance and heat resistance of the resin become worse when the phosphorous compounds are used, so that there is a possibility that the safety reliability of the printed wiring boards at the time of their manufacturing process and their usage is deteriorated. Further, the resin composition becomes hard and brittle when the metal hydroxide is used and the peel strength between the resin and the copper foil is substantially reduced, consequently there has been a problem that faulty adherence occurs between the base material and the copper foil.
Therefore, it has been required to be able to form a resin layer of resin applied-copper foil employing a resin compound which is free of halogen elements and to manufacture a copper-clad laminate without any of above described problems, and to provide high flame retardancy, excellent water resistance and heat resistance, and good peel strength between a base material and copper foil.
Hence, the inventors have developed a resin compound which will be described hereinafter and also developed a resin sheet and resin applied-copper foil which use this resin compound as a result of our intensive studies. Further, manufacturing a copper-clad laminate by the use of above described materials allows a printed wiring board to be provided which is excellent in both characteristics, that is, heat resistance and flame resistance.
Firstly, the present invention as set forth in claim 1 is a resin compound for fabricating an interlayer dielectric of a printed wiring board, wherein the resin compound has a composition comprising an epoxy based resin which includes an epoxy resin curing agent having 5 to 25% by weight of nitrogen and maleimide compounds which have thermosetting properties (possible to limit the content?)and being free of halogen elements, and that the resin compound is formed by dissolving the composition in an organic solvent.
According to the present invention, when the nitrogen content in the curing agent within components of the epoxy based resin which constitutes the resin compound for fabricating the interlayer dielectric is 5 to 25% by weight, good performance can be delivered in terms of the flame retardancy and the water absorption, so that the selective usage of proper curing agent for the epoxy based resin allows the cured epoxy resin to have a self-digesting characteristic. In addition, the present invention is to provide a resin compound for fabricating an interlayer dielectric having higher heat resistance and flame retardancy, in combination with the use of maleimide compounds having higher flame retardancy by itself with a thermosetting property. Further, this composition makes it possible to eliminate halogen elements represented by chlorine etc. which are added to the compound in order to impart the flame retardancy, so that it becomes possible to reduce loads to the natural environment in a stage of discarding the copper-clad laminate and the printed wiring board.
The resin compound for fabricating the interlayer dielectric as set forth in claim 1 is described as one basically comprised of the epoxy based resin and the maleimide compounds. The epoxy based resin as described herein means a resin which uses epoxy resins as a base material and contains at least a resin curing agent required for curing the epoxy resin, and is also assumed to include other resin components such as melamine resin or phenol resin, an accelerator, an antifoaming agent, and a leveling agent, etc. which are added as necessary. These additional components can control the curing rate of the resin and produce an effect of the cost reduction. This is because, as a general concept in the chemical context, it becomes impossible to readily conduct the curing of the epoxy resin if the epoxy resin curing agent is not included. The maleimide compounds mentioned in the present invention will be described in detail in a description of the resin compound for fabricating the interlayer dielectric as set forth in claim 2.
In claim 1, a sentence xe2x80x9ccomprising an epoxy based resin which includes an curing agent having 5 to 25% by weight of nitrogen and maleimide compounds which have thermosetting propertiesxe2x80x9d is described. This sentence is described as a concept including not only a resin compound for fabricating an interlayer dielectric obtained by mixing xe2x80x9can epoxy based resin which includes a curing agent having 5 to 25% by weight of nitrogenxe2x80x9d with xe2x80x9cmaleimide compounds which have thermosetting propertiesxe2x80x9d each of which is separately produced but also a resin compound for fabricating an interlayer dielectric obtained by simultaneously adding the maleimide compounds when the epoxy resin and the above described epoxy resin curing agent, etc. are dissolved. That is, there is no particular restrictions on the order of mixing the constituent compounds when producing the resin compound used for fabricating the interlayer dielectric.
A solvent used for the dissolution will be described in the following description of a method for manufacturing. The above described resin compound for fabricating the interlayer dielectric is used for forming an insulating resin layer between the copper foil circuit layers of the copper-clad laminate.
In addition, as defined in claim 2 that the resin used for fabricating the interlayer dielectric of the printed wiring board as set forth in claim 1, wherein the epoxy based resin comprises epoxy resins having two or more glycidyl groups per molecule, polymers having crosslinkable functional groups within a molecule, a crosslinker which is added as necessary, and a phenol novolak epoxy resin curing agent containing triazine rings within a molecule, it is desirable to use the resins described above as epoxy resins used in the present invention.
An epoxy resin having two or more glycidyl groups per molecule is used as the epoxy resin. If an epoxy resin having one glycidyl group per molecule is used, a crosslinking state of the resin tends to be insufficient and results in lowering of the flame retardancy, so that an object of the present invention cannot be achieved. Although an epoxy resin which is generally used for electric and electronic materials can be used without any particular restrictions as far as the above described condition is satisfied, an epoxy resin which is substantially free of halogens is preferable to be used in this context.
An epoxy resin having two or more glycidyl groups per molecule is desirably 20 to 70 parts by weight given that the total amount of the resin compound is 100 parts by weight. If the blending proportion of the epoxy resin is less than 20 parts by weight, the adhesive property between the copper foil and the base material after being processed to the copper-clad laminate becomes lower and the peel strength also tends to be decreased. By contrast, if the blending proportion of the epoxy resin is 70 parts by weight or more, flowablilty of the resin compound becomes too much to treat the compound, so that it becomes difficult to control a layer thickness when a resin layer is formed on the copper foil or a resin sheet described below is manufactured.
Therefore, as defined in claim 3 that the resin used for fabricating the interlayer dielectric of the printed wiring board as set forth in claim 2, wherein the epoxy resins having two or more glycidyl groups per molecule are free of halogen elements and are any one or more of bisphenol A epoxy resin, bisphenol F epoxy resin, novolak epoxy resin, cresol novolak epoxy resin, and glycidylamine epoxy resin, the epoxy resins described herein which are free of halogen elements are most preferable to be used. These are capable of exploiting the most stable substrate performance from a broad view in an application for manufacturing the printed wiring board, in terms of not only the flame resistance but also various characteristics including the peel strength between the base material and the copper foil and the chemical resistance.
xe2x80x9cPolymers having crosslinkable functional groups within a moleculexe2x80x9d in a sentence xe2x80x9cpolymers having crosslinkable functional groups within a molecule and a crosslinker which is added as necessaryxe2x80x9d as set forth in claim 2 is used for preventing cracks from occurring in a resin layer when the resin compound is coated on the copper foil to form the resin layer on the copper foil and processed to a resin applied-copper foil, preventing resin powder from scattering at the time of cutting and crushing after being processed to the copper-clad laminate, and ensuring a certain range of flowability by performing the viscosity control of the resin compound before forming the resin layer.
As for the polymers, as defined in claim 4, it is preferable to use any one or more of polyether sulfone resin having a hydroxyl group at a terminal, polyvinyl acetal resin having repeated hydroxyl groups within a molecule, and phlenoxy resin, as the polymers which have crosslinkable functional groups within a molecule.
There is not necessary to particularly limit the polymer as described above as far as the polymer corresponds to the above described polyether sulfone resin having a hydroxyl group at a terminal, polyvinyl acetal resin having repeated hydroxyl groups within a molecule, or phenoxy resin. If a sufficient crosslinking reaction can be allowed to proceed by the use of polymer with crosslinkable functional groups within a molecule such as the hydroxyl groups in the polymers described above, highly excellent heat resistance can be imparted to the copper-clad laminate or the printed wiring board being manufactured by using thereof, even when the resin compound is cured and becomes a part of the copper-clad laminate. Therefore, in case of using a resin compound which uses a polymer having no crosslinkable functional groups within a molecule, the heat resistance after curing the resin compound becomes significantly lower, so that the compound may not be used.
In addition, the polymer having crosslinkable functional groups within a molecule is desirable to be in a range of 5 to 30 parts by weight given that the total amount of the resin compound is 100 parts by weight. The effects of preventing cracks from occurring and preventing the resin powder from scattering cannot be produced when the polymer is less than 5 parts by weight, whereas the flowability of the resin becomes too low when the polymer is 30 parts by weight or more, so that controlling the layer thickness becomes difficult when the resin layer is formed on the copper foil or when a resin sheet described below is manufactured, as in the case where the amount of epoxy resin becomes excessive.
xe2x80x9cA crosslinker which is added as necessaryxe2x80x9d is used depending on a kind of polymer, that is, blocked isocyanate is used for bisphenol A epoxy resin for example and other crosslinkers should be considered to be used as appropriate, so that there is not necessary to impose particular limitations on the above described crosslinkers. In addition, the present invention does not always use the crosslinker. Because, in case of using polyether sulfone resin as a polymer for example, a crosslinking reaction with the epoxy resin can be conducted without adding the crosslinker separately.
xe2x80x9cAs a phenol novolak epoxy resin curing agent containing triazine rings within a moleculexe2x80x9d as set forth in claim 2, one comprised of one or two of melamine and benzoguanamine and a compound obtained from a condensation reaction of phenols and formaldehyde, and having 5 to 25% by weight of nitrogen content is most preferably employed as defined in claim 5. More specifically, the curing agent is, for example, an AT novolak resin having a trade name of LA-7054 manufactured by Dainippon Ink Inc. The resin compound according to the present invention, which uses the above described compounds as a curing agent of the epoxy resin, provides a self-digesting characteristic.
The usage amount of the phenol novolak epoxy resin curing agent which contains triazine rings within a molecule should not be specified to a certain amount which is particularly limited because this amount is calculated from an equivalent ratio to the epoxy resin to be used and from experimental data.
As maleimide compounds, it is most preferable to use maleimide compounds having thermosetting properties as set forth in claim 6. That is, the compounds are, for example, N,Nxe2x80x2-(diphenylmethane)bismaleimide, bis(3-ethyl-5-methyl-4-maleimide)methane, and 2,2-bis[4-(4-maleimidephenoxy)phenyl]propane. Still further, it is possible to obtain good results by the use of a thermosetting maleimide compound which can be obtained by Michael addition reaction of the above described maleimide compound with polyamides.
A content of the maleimide compound having a thermosetting property in the resin compound according to the present invention is desirable to be 10 to 50 parts by weight given that a total amount of the resin compound is 100 parts by weight. If the maleimide compound is less than 10 parts by weight, an intended sufficient flame retardancy cannot be obtained when the resin compound according to the present invention is cured as an insulating layer of the copper-clad laminate. By contrast, if 50 parts by weight or more of the maleimide compound is used, the cured resin becomes extremely brtittle and its mechanical characteristics also become worse, so that the reliability for shock resistance or load capacity etc. of the resultant printed wiring board is compromised. Such a worsening in the mechanical characteristics becomes an extremely critical defect for a printed wiring board to which a load of a flyback transformer having several kilograms of loads is directly applied, such as a mother board being built in a television for example.
The resin compound according to the present invention as described above, which is used as a material for fabricating the interlayer dielectric of the copper-clad laminate, makes it possible to impart excellent heat resistance and flame resistance to a printed wiring board which is obtained from the copper-clad laminate.
As defined in claim 7, a method for producing the resin compound used for fabricating the interlayer dielectric of the printed wiring board as set forth in any of claims 1 to 6, wherein a composition is made to have 20 to 70 parts by weight of epoxy resins, 5 to 30 parts by weight of polymers having crosslinkable functional groups within a molecule, 10 to 50 parts by weight of maleimide compounds having thermosetting properties, and a balance of a crosslinker added as necessary and a phenol novolak epoxy resin curing agent containing triazine rings within a molecule given that a total amount of the resin compound excluding a solvent is 100 parts by weight, and that a solids content after the composition is added to and dissolved in the solvent becomes 30 to 80% by weight, can be derived from the blending proportion used for obtaining the resin compound according to the present invention.
The meanings of blending proportion of respective resin components and a curing agent etc. are as described above. And a reason for defining as xe2x80x9ca solids content after the composition is added to and dissolved in the solvent becomes 30 to 80% by weightxe2x80x9d in this claim 7 is that a proper viscosity range which is capable of favorably controlling the resin layer thickness can be obtained within this range when a resin applied-copper foil is formed or a resin sheet is formed. That is, if the solids are less than 30% by weight, the viscosity of the resin becomes lower and its flowability becomes too large, so that a coating thickness of the resultant resin layer is excessively reduced. By contrast, if the solids exceed 80% by weight, the viscosity of the resin becomes higher and its flowability becomes too low, so that a coating thickness of the resultant resin layer is excessively increased.
In addition, as a solvent used for obtaining the resin compound according to the present invention, as defined in claim 8, it is preferable to use a mixed solvent of N-methylpyrrolidone and methyl ethyl ketone which has a mixing ratio of N-methylpyrrolidone/methyl ethyl ketone=50/50 to 40/60 (by weight).
That is, as a solvent generally used for dissolving a resin according to the present invention, it is desirable to use a single component such as methyl ethyl ketone which is readily volatilized during a drying process. However, constituents of the resin according to the present invention include maleimide compounds, so that it is almost impossible to dissolve the compounds with the use of methyl ethyl ketone only. Therefore, the inventors have contemplated the use of the mixed solvent of N-methylpyrrolidone and methyl ethyl ketone.
According to a comparison of a property of this methyl ethyl ketone with that of N-methylpyrrolidone, a boiling point of the methyl ethyl ketone is 79.6xc2x0 C., whereas a boiling point of the N-methylpyrrolidone is around 200xc2x0 C. Thus, it can be said that the N-methylpyrrolidone is more resistant to be volatilized than the methyl ethyl ketone. For these reasons, a usage amount of N-methylpyrrolidone should be within a certain range in which the maleimide compounds used for the present invention can be dissolved and should be minimized.
As mentioned above, the inventors have decided as a result of our intensive studies that the maleimide compounds used for the present invention can be readily dissolved in the mixed solvent which includes a certain amount of N-methylpyrrolidone, that is, a ratio of N-methylpyrrolidone/methyl ethyl ketone in the mixed solvent is at least 40/60 (by weight). If the ratio of N-methylpyrrolidone/methyl ethyl ketone exceeds 50/50 (by weight), a volatilization speed which is usually required is not satisfied, so that the industrial productivity is not satisfied.
The following method is contemplated when the interlayer dielectric of the copper-clad laminate is fabricated by the use of these resin compounds. A resin sheet which is processed in a semi-cured state is inserted between an inner layer base material and outer layer copper foil as in the case of employing a prepreg, then subjected to the hot press molding in order to be able to obtain a copper-clad laminate. Therefore, claim 9 defines that the resin compound for fabricating the interlayer dielectric of the printed wiring board as set forth in any of claims 1 to 6 is a resin sheet in a semi-cured state for forming an insulating layer for manufacturing the copper-clad laminate. Further, claim 11 defines that the copper-clad laminate is manufactured by the use of the resin sheet for forming the insulating layer as set forth in claim 9.
The method for processing the resin compound into the resin sheet may be a method for processing the resin compound according to the present invention being in a semi-cured state into a sheet or a method wherein a resin layer having a predetermined thickness is formed on a plastic film as a carrier, and the carrier is peeled off at the time of usage.
Further, claim 10 defines that the resin applied-copper foil with the resin layer formed on a surface of the copper foil by the use of the resin for fabricating the interlayer dielectric of the printed wiring board as set forth in any of claims 1 to 6, and claim 12 defines that the copper-clad laminate is manufactured by the use of the resin applied-copper foil as set forth in claim 10.
The above mentioned matters are claimed because the resin compound according to the present invention can be used for manufacturing the resin applied-copper foil and this resin applied-copper foil can be used for manufacturing the copper-clad laminate.
As for the manufacture of the resin applied-copper foil, a semi-cured resin layer is formed on a surface of the copper foil by coating the resin compound according to the present invention on an adhesive surface of the copper foil to the inner layer base material, with the use of the resin coating apparatus referred to as so-called an edge coater or a roll coater, to a predetermined thickness, then drying the resin compound.
Using this resin applied-copper foil like the common copper foil, it becomes possible to manufacture the copper-clad laminate without the use of materials for fabricating the insulating layer such as a prepreg.
Thus obtained copper-clad laminate, since framework materials are not included as in the case of the FR-4 prepreg, is excellent in surface smoothness, possible to provide good registration, easy to form fine pitch circuits, and excellent in formation accuracy of via holes through the laser machining, and also excellent in heat resistance and flame resistance.
The present invention is described below in more detail, while describing the examples according to the present invention.